audi_bhoy : indeed, it came out, I didn't apply enough strength on it, lol. The rest of the piston came out as expected and nothing posed a problem. I was careful as you told me, and it fires correctly.
I've made pictures of the new screws :
KJW customer service told me that yes, they've added those because there was some cases where people told them platic guide would "jump out". As for the hexagonal screw, it just prevents the nozzle from going too far out.
Talking nozzle... Did anyone with a V2 notice it staying a little "stuck" for a milliseconde in the hop up unit ? I've made a photo to illustrate (I wasn't firing, just pulled back the bolt a little, but it looks like that)
But the spring in the rear does its work and eventually the nozzle is pulled back into the carrier. Does it mean it's just the plastic that needs a little use-wearing ? Or is something wrong ? I've cleaned the nozzle and the hop up, but it still happens. Dismantled everything, cleaned, and reassembled.